1. Field of the Invention
The present invention relates to an improved spring design for an article of manufacture and, more specifically, the present invention relates to an improved spring design constructed of a bi-textured strip of materials which exhibit different amounts of irradiation induced strain for use in the core of a nuclear reactor.
2. Description of the Prior Art
The basic principle of concern of the present invention is that differential irradiation-induced growth strain will cause internal stresses to be generated in a spring. Distortion of nuclear reactor components occurring from this cause is a well known effect, and a fundamental problem which must be faced in the design of nuclear reactor cores. This type of problem is analogous to the problems which arise in the design of components due to stresses and strains caused by differential thermal expansion. There have been methods proposed to mitigate the effects of such differential growth in a radiation environment, but these have generally failed to achieve the results realized by the present invention.
FIG. 1 is a schematic illustration of the application of a conventional flat strip spring which is used to hold tubular fuel elements in place in a nuclear power reactor. FIG. 1(a) shows the spring 1 in an unloaded position. However, in FIG. 1(b), the spring 1 has been deformed elastically by inserting the fuel rod 2. This fuel rod 2 establishes membrane stresses through the cross-sectional area of the spring 1. The force F that the spring 1 exerts on the rod 2 will exist so long as these membrane stresses are maintained. During operation at high temperatures in the irradiation field of a nuclear power reactor, the stresses cannot be maintained due to the presence of accelerated stress relaxation.
As the stresses relax, the force exerted by the spring on the rod decreases. When this force falls below an acceptable design value, the rods will no longer be held securely, with the result that fretting, chattering and excessive wear of the rods can occur due to flow induced vibrations and other causes. Moreover, if the fuel rods are not properly spaced, nuclear and thermal performance of the reactor fuel assemblies can be adversely affected.
As discussed above, the design life of the conventional spring is limited by the degree of stress relaxation occurring. It is difficult to provide positive assurance that the required minimum force will still exist at the end of the core life with such conventional springs since data indicate stresses can relax to close to zero in the reactor environment. Although the springs are occassionally constructed of high strength materials (e.g. steel) to provide a high initial stress to obtain a longer life, often these materials are not compatible with the nuclear core design.
From the foregoing, it is clear that the need exists for an improved spring design for use in the core of a nuclear reactor which can exhibit varying amounts of irradiation-induced strain, producing bending stresses which reduce or eliminate the irradiation-induced relaxation of the desirable initial stresses loaded in the spring.